I wander into the room next to the London Sofitel's Peggy Ashcroft Suite, where I am to interview Sophie Marceau. The vast bed is tempting, as is the room service menu. Should I order Marceau something from the rotisserie? There's no time to decide. Marceau walks into the room impeccably groomed and dressed to talk about a film in which, finally, she does not play a sexpot driving men crazy, which has been the role she's played almost incessantly since, at the age of 14, she made her debut in the 1980 proto-brat-pack teen flick, La Boum (The Party) - a performance that resulted in her first magazine covers, a million-franc contract with Gaumont and soap commercials that made her a star in Japan.

Instead, the 41-year-old actor is here to be interviewed about her performance in Female Agents as a secret operative fighting the Nazis. It's an unexpected role for Marceau to take: it's tough stuff, but unintentionally hilarious. Were the women of the French Resistance so glamorous? Did they really fight in full makeup? Doesn't Marceau look hot as she takes aim with steam trains puffing and blowing all around her?

Like last year's Days of Glory, which dealt with the group of Francophone north Africans who helped throw the Nazis out of France, Female Agents is one of those films that rather worthily tackles a neglected theme in the Gallic history of the second world war. Female Agents is about the role of women fighters under Nazi occupation. The film's original title, Les Femmes de l'Ombre (Women of the Shadows), makes this clear, since it plays on the title of Jean-Pierre Melville's 1969 film about the French Resistance, L'Armée des Ombres (Army of Shadows). It's a serious film, so my questions had better be serious, too.

Imagine, I say to Sophie Marceau, that your brother is being tortured by Nazis. He knows a secret that, if revealed, would tell Hitler precisely where on the Channel coast the invading allied armies plan to land. So far, he has heroically kept silent, but now they have captured you too. If he doesn't tell them what they want to know, he will have to see you die at the hands of these brutes. What would you want him to do - confess or see you suffer? Should he risk the liberation of Europe from Nazi oppression to save his sister's skin?

Marceau flicks her hair over her shoulder and looks contemplatively out the window on to a lovely London afternoon. She has a beautiful profile, brown eyes that you might write poetry about and lips curved into an ironic smile. "These situations are so tragic," she replies. "It's not a funny game, but the kind of terrible thing that happens often in war." That is true: it's why, no doubt, the director Jean-Paul Salomé put such a scene in Female Agents, in which Marceau plays the sister and Julien Boisselier the brother.

"I think I'm a tough person, but for sure, I would talk under torture," says Marceau. "The body can't deal with a certain level of suffering." Fair enough, but what would you want your brother to do? "I think the wonderful thing about what happens in this film is that the brother acts out of love," says Marceau.

In the film, the brother decides his and his sister's fate. Marceau empathises with that reaction. "He does what a human being can do. At the last moment, it's to be human. It's not to be systematic. At the end, you see a kind of humanity, a kind of love." And you like that? "I believe in causes, but sometimes it's better to be simply human. But there is no right answer."

Marceau has been thinking a lot about war and torture recently. "These are not just old historical questions. They are with us right now - the denial of human rights, the existence of torture in so-called civilised countries, sending men to their deaths. I've just remembered a scene I did when we were filming Anna Karenina in St Petersburg. [She was the eponymous heroine in the 1997 film adaptation of Tolstoy.] There was a scene at a railway station, and behind us there was a group of young people dressed like soldiers in coats that were too big. These guys were like my sons. They were going to fight. It's something unbelievable." What's unbelievable? "That such young men were sent to do such things. Now, I think about these things with the eyes of a mother. How I would feel if my son was sent to war?"

Marceau has two children: a son, Vincent, born in 1995 to her then husband, the film producer Andrzej Zulawski, and a daughter, Juliette, born in 2002 to the film producer Jim Lemley. "You never know what you would be like in a war. I play a woman who has just lost her husband, who is pregnant, and who loses all of her comrades." It's worse than that, I point out: at the end of the film we are told that the woman whose life story you have been interpreting dies childless in 2004. "Yes, that's so sad. She lost her child before it was born. It's better to lose a child when you're pregnant than when it has been born, I suppose. But I understand why women would want to become pregnant amid the horrors of war. I know people in world war two who had children. They said to me, 'If we didn't have children, we might never have carried on.' Having children gave them a reason to carry on."

Is it unusual for you to take on such a serious role? "I don't know about that. Today I act in all kinds of different roles - some serious, some not. I've just finished filming a teen picture." Like La Boum, 28 years later? "Yes, except I play the mother. And it is not a comedy." The film is called Lol, a picture in which she plays the mother to glamorous but adorably dysfunctional Parisian teens. Its director, Lisa Azuelos, told Madame Figaro magazine: "Sophie incarnates the ideal mother with whom all women can identify." How does it feel to play an ideal mother rather than a sexpot? "Oh, I don't believe that I play an ideal mother! All I will say is that I'm playing a lot of different roles that, to be honest, are more interesting and demanding than any I've played before."

Among them is the long-awaited second film by Marina de Van, the director whose shocking debut In My Skin (Dans Ma Peau), about a woman who becomes obsessed with self-mutilation and, later, self-cannibalism, is to be followed by another film about a woman's identity crisis. During the film, Ne Te Retourne Pas (Don't Come Back), Marceau mutates into Monica Bellucci, which will be worth seeing. "The character I play finds out that she is not the person she thought she was. She's going to find a photograph at her mother's house of a woman in Italy. She's going to go to Italy and there finds she changes into another woman, Rosa Maria, who is played by Monica. I completely metamorphose myself. There are lots of special effects. It's very disturbing." To watch or to play? "Both, I should think. It's certainly uncanny to play such a role where you feel yourself slipping away from you. The only parallel I can think of is when you speak another language, you become a different person."

Marceau is used to such metamorphoses. It started when she changed her surname because Marceau sounded better than Maupu for the screen, and it hasn't stopped mutating since. "Having been an actor for so long, you have to occupy another person's world again and again. And, having started very young, I have been through this identity crisis again and again. It becomes your life, you know? It's not really a career, more like a series of identity crises."

So why do it? "I once said that it was like therapy, except you get paid to do it rather than having to pay for it. I still think that's true." I remind her that her one foray into writing was a faux memoir of a glamorous actress called Telling Lies. Are you telling the truth now? She shrugs-pouts delectably. "Yes, of course. But you know it's true that I'm working a lot. I don't have time to digest what I'm doing."

There's time for one last question before Marceau takes the Eurostar home. What, as an actor, was the worst identity crisis you went through? Was it being a Bond girl in The World Is Not Enough opposite Pierce Brosnan or being on the receiving end of Braveheart's ardour? "I wasn't a Bond girl," she corrects me. "I was the villain. I played a megalomaniac, which might surprise you was a very happy experience. I felt sometimes too responsible as an actor because people promote violence or weird things that I don't want to be part of. But this time it wasn't like that. It's fun because you know you're not harming anybody.

"That," she says as she rises to her feet, heads for the door and favours me with her other profile, which is just as lovely as the first, "is always what I hope. That no one gets harmed because of what I do."